FIG. 1A is a simplified block diagram showing an envelope restoration (ER) transmitter (TX) 1 architecture that includes an amplitude modulation (AM) chain and a phase modulation (PM) chain. Bits to be transmitted are input to a bits to polar converter 2 that outputs an amplitude signal, via propagation delay (PD) 3, to an amplitude modulator (AM) 4. The AM 4 (after digital to analog conversion) supplies a signal for controlling the output level of a TX power amplifier (PA) 6 through the use of a controllable power supply 5. The bits to polar converter 2 also outputs a phase signal via propagation delay 3 to a frequency modulator (FM) 7, which in turn outputs a signal via a phase locked loop (PLL) 8 to the input of the PA 6. The transmitted signal at an antenna 9 is thus generated by simultaneously using both phase and amplitude components. The benefits that can be gained by using the ER transmitter architecture include a small size and an improved efficiency, as compared to many conventional techniques.
Evolving digital cellular systems such as those known as Enhanced Data Rate for GSM Evolution (EDGE) and Wideband Code Division Multiple Access (WCDMA) use modulation techniques with non-constant envelopes. This makes the use of traditional transmitter architectures having a linear power amplifier (PA) inefficient and, as a result, transmitter architectures with saturated or switched mode PAs have recently become of greater interest.
One such an approach is the Kahn Envelope Elimination and Restoration (EER) technique, which combines a highly efficient but non-linear RF PA with a highly efficient envelope amplifier. General reference may be had, as an example, to U.S. Pat. No. 4,688,255, Aug. 18, 1987, “Compatible AM Broadcast/Data Transmission System”, by Leonard R. Kahn.
As is shown in FIG. 1B, which is a simplified depiction of FIG. 1A, in the Kahn EER transmitter 1 the RF signal is split into a PM and an AM signal by the transmitter modulator 2. The PM signal is directly amplified by a saturated PA 6, while the supply voltage Vccrf of the PA 6 is modulated by the AM signal via amplitude modulator 5, which is provided with the battery voltage Vbatt. Modulating the supply voltage of the PA 6 has the effect of amplitude modulating the transmitted PM carrier signal
The efficiency of the Kahn EER transmitter 1 is defined by the performance of the amplitude modulator 5 and the PA 6. The efficiency of the saturated or switched mode PA 6 may be greater than 70% at peak-envelope power (PEP), and it may remain high also during a back-off condition. From the several possible approaches for the realization of the amplitude modulator 5 the simplest is a linear regulator (e.g., a PNP transistor), which also provides the widest bandwidth. However, the linear regulator has poor efficiency at low envelope voltages, which tends to degrade the system performance.
From an efficiency point of view the class-S modulator (similar to a fast dc/dc-converter) is more suitable, although it is more complex. The efficiency of the class-S modulator 5 can be greater than 90%, although this figure is reduced at high clock frequencies.
By way of an introduction to the problems solved by the use of the embodiments of this invention, FIG. 2 shows a simplified block diagram of an class-S modulator 10 that feeds a switched mode power amplifier 12. The class-S modulator 10 includes transistors Q1 and Q2 driven by a driver 10A and a (fixed frequency) pulse width modulator (PWM) 10B. The class-S modulator 10 functions as a two-pole switch to generate a rectangular waveform with a switching frequency several times (typically five to seven times) that of the output signal.
General reference with regard to the use of a class-S modulator with a Kahn EER transmitter can be made to pages 40 and 42 of a publication entitled, “RF and Microwave Power Amplifier and Transmitter Technologies—Part 3”, High Frequency Electronics, September 2003, pages 34-48. Reference may also be made to U.S. Pat. No. 6,049,707, Apr. 11, 2000, “Broadband Multi carrier Amplifier System and Method Using Envelope Elimination and Restoration”, by Buer et al.
The selection of the components L1, C1 of a low-pass filter 10C is a compromise between passing a desired envelope and rejecting spurious components that are inherent in the PWM process. When using the same configuration for the systems with different modulations (e.g. GSM/EDGE) the bandwidth is defined by the modulation having the widest AM-bandwidth. This leads to a non-optimum solution for the narrow-band modulation because it is possible to increase the efficiency by reducing the clock frequency and the bandwidth of the low-pass filter 10C.
From the PA 12 point of view the stability at low frequencies is the important issue if the total value of a bypass capacitance Cb at a power supply line or rail is restricted. A typical approach may use several parallel capacitors beginning with a picofarad (pF) range capacitor for RF-frequencies and ending with a value of approximately one microfarad (microF). The most critical frequencies for are typically in the range of about 10 MHz to 500 MHz, and the stability is guaranteed by a proper choice of the RF-choke Lc (typically a few nano-Henries, nH) and bypass capacitor (typically 1 nF to about 100 nF). However, in the Kahn architecture the maximum value of the bypass capacitor Cb is restricted by the envelope bandwidth to a range of about 100 nF to about 200 nF. This can cause difficulties, especially in the case where the same class-S modulator 10 is supplying several power amplifiers (low/high band).
The use of the fixed low-pass filter 10C, as shown in FIG. 2, is disadvantageous at least of the reason that its bandwidth is defined by the modulation that has the broadest AM-bandwidth, resulting in a non-optimum filtering solution for those signals with a narrower bandwidth.